A conventional linear ion trap typically includes two or more poles, each of which includes two or more rods. The rods in an ion trap collectively form a rod set or rod array. In a conventional linear ion trap, the rods are parallel to a longitudinal axis of the ion trap. The longitudinal axis lies along a Z-dimension. A plane normal to the Z-dimension lies on an X-Y plane, defined by orthogonal X and Y dimensions. In a linear ion trap with four rods, two opposing rods are typically defined as X pole rods and are spaced apart equidistant from the longitudinal axis in the X dimension. The X pole rods form an X pole. The other two opposing rods are typically defined as Y pole rods and a spaced apart equidistant from the longitudinal axis in the Y dimension. The Y pole rods form a Y pole.
To function as an ion trap, the parallel rod set is augmented with end caps or lenses that supply an axial trapping potential.
An RF potential is applied to the X and Y poles. Typically, the RF potential is equal in magnitude and frequency, but out of phase by 180°. The end caps provide fringing fields. Some ions, depending on the characteristics of the radial trapping potential, are trapped within the rod set, while others are radially ejected.
Ions are ejected, for the purposes of mass analysis, either radially, through one or more rods, or axially, through the process of mass selective axial ejection (MSAE). In the MSAE technique ions are first excited radially to a high fraction of the field radius, r0 defined above, and then, through interaction with the fringing fields at the exit of the ion trap, are detected axially.